AC Bridges Questions and Answers

Answer: C

Hay's bridge balances an inductive arm against a capacitor with a series resistance and suits high-Q coils; Schering's gives capacitance and loss tangent at high voltage; and the Wien bridge is the one whose balance depends on frequency and therefore measures it. Each of the three is named after the man who arranged the arms in that particular way.

Answer: D

Hay's bridge needs a large Q for its inductance expression to be exact; Anderson's handles lossy coils with only a fixed standard capacitor; and Owen's separates the two balance conditions so that each may be set without disturbing the other.

Answer: A

The detector is first put on the earth point and the Wagner branch adjusted, then returned to the bridge and the arms adjusted. Each disturbs the other, so the pair of operations is repeated until both nulls hold at the same time.

Answer: B

Correct answer: 1 and 2 only are correct

Statements 1 and 2 are correct: the Maxwell–Wien bridge measures an unknown inductance against a standard capacitance, and its balance conditions contain no frequency term, so the supply frequency need not be known or held constant. Statement 3 is wrong - balancing it requires a variable resistor whose value grows with the coil’s quality factor, which becomes impractical above about ten, so a high-Q coil is measured on the Hay bridge instead.

Answer: C

A moving coil movement averages an alternating quantity to zero and so cannot indicate unbalance; a vibration galvanometer is mechanically resonant and therefore selective; and an oscilloscope shows the waveform, from which the phase of what remains tells the operator which control to move.

Answer: D

Screening does nothing for lead resistance, which is dealt with by short thick connections or by a four-terminal arrangement. Fixing the strays, excluding the operator's own capacitance and preventing magnetic coupling are all genuine purposes of the screens.

Answer: A

Maxwell's inductance bridge compares the unknown coil directly with a calibrated variable inductor, so no capacitor appears in it at all. Every other bridge listed uses a capacitance standard, which is the usual and better practice where it can be arranged.

Answer: B

Thermal emf is a direct voltage and troubles direct current work, particularly low-resistance measurement, where the current is reversed to cancel it. An alternating detector ignores it entirely. Strays, mutual inductance and residuals are the characteristic alternating current difficulties.

Answer: C

A residual sound or deflection that no setting removes points to something outside the four arms: harmonic content that cannot balance at the fundamental setting, or a stray capacitance injecting current into the detector. A tuned detector and a Wagner earth address the two cases.

Answer: D

The inductance given by Hay's bridge carries a factor containing \( 1/Q^2 \), which is negligible for a Q of fifty but amounts to a quarter at a Q of two. At such a value Anderson's or the Maxwell-Wien bridge should be used instead.